1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing tyres.
2. Description of the Related Art
A tyre for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures, integrated into the regions usually identified as “beads”.
Associated with the carcass structure is a belt structure comprising one or more belt layers, disposed in radial superposed relationship with respect to each other and to the carcass ply and having textile or metallic reinforcing cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. Applied to the belt structure at a radially external position is a tread band also made of elastomeric material like other semifinished products constituting the tyre.
It is to be pointed out, to the aims of the present description, that by the term “elastomeric material” it is intended a composition comprising at least one elastomer polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers, for example. Due to the presence of the cross-linking agents, this material can be cross-linked through heat so as to form the final manufactured article.
Respective sidewalls of elastomeric material are also applied to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads. In tyres of the tubeless type, an airtight coating layer, usually called “liner” covers the inner surfaces of the tyre.
Generally, in the manufacture of tyres for vehicle wheels it is provided that subsequently to building of the green tyre through assembly of the respective components, a moulding and curing treatment be carried out which aims at determining the structural stabilisation of the tyre through cross-linking of the elastomeric compositions and also at forming a desired tread pattern thereon, as well as possible graphic distinctive marks at the tyre sidewalls.
Manufacturing processes of recent conception are known in which building of the green tyre is carried out by making the different components thereof on a rigid toroidal support, the conformation of which is coincident with the inner conformation of the finished tyre. For achieving the moulding and curing treatment, the green tyre is enclosed in the moulding cavity of a vulcanisation mould of a shape matching the outer conformation to be given to the finished tyre, together with the toroidal support on which the tyre itself has been build.
WO 01/00395, in the name of the same Applicant, discloses use of a toroidal support having an outer diameter slightly smaller than the inner diameter of the finished tyre. The crown portion of the tyre is moulded against the inner surface of the moulding cavity following a radial expansion induced by steam under pressure introduced into a diffusion interspace defined between the toroidal support and the inner surface of the tyre itself.
U.S. Pat. No. 5,853,526 discloses building of a tyre carried out through formation of the different components on an expandable toroidal support comprising a reinforced bag the inner end flaps of which are sealingly fastened to mutually coaxial anchoring flanges which are integral with two half-shafts telescopically in engagement with each other. The reinforced bag, inflated to a predetermined pressure, keeps a predetermined geometric structure corresponding to the inner conformation of the tyre to be built, so that it lends itself to support the components of the tyre being processed. When building has been completed, the tyre is shut up in the vulcanisation mould together with the expandable toroidal support. The reinforced bag is adapted to receive steam or other working fluid under pressure, to cause pressing of the tyre against the inner surfaces of the moulding cavity and simultaneous heat supply to the tyre itself to achieve vulcanisation.
In practical implementation of said processes, the Applicant could observe some difficulties correlated with the moulding and curing treatment of the tyre.
In a tyre vulcanisation and moulding process as the one disclosed in WO01/00395, an “imposed-volume” moulding is required on wide regions of the tyre, by way of indication extending from the tyre beads along the sidewalls, until the shoulders.